1. Field of the Invention
The present invention relates to a fuel vapor control valve device adapted to be fitted between a fuel tank and a canister for storing fuel vapor therein.
2. Description of Related Art
As a valve device of the kind specified, there is known a fuel vapor control valve arrangement comprising a canister passage communicating with a fuel tank, a first communication passage and a second communication passage which connect the canister passage and the fuel tank passage, a constant-pressure operating valve for opening and closing the first communication passage, and a differential pressure operating valve for opening and closing the second communication passage, the arrangement being made such that the constant-pressure operating valve opens the first communication passage when the internal pressure of the fuel tank exceeds a predetermined value, thereby allowing the fuel vapor to flow from the fuel tank into the canister, and the differential pressure operating valve opens the second communication passage only when the internal pressure of the canister becomes higher than that of the fuel tank and the difference between the internal pressure of the canister and that of the fuel tank exceeds a predetermined value, thereby allowing the fuel vapor to flow from the canister back into the fuel tank. Further, the constant-pressure operating valve is of an absolute pressure operating type adapted to be actuated on the basis of the atmospheric pressure wherein its valve body is formed by a diaphragm, one side of which faces the first communication passage and the fuel tank passage, and the other side of which is open to the atmosphere.
In case of the above-mentioned construction, however, the constant-pressure operating valve and the differential pressure operating valve are arranged to be independently and automatically opened and closed in response to change in the internal pressure of the fuel tank. Therefore, when the internal pressure of the fuel tank is less than a predetermined value, there occurs a condition that although the constant-pressure operating valve is closed, the fuel tank is filled with fuel vapor to some degree. If a lid closing a filling port of the fuel tank is opened in this condition, for example, for refuelling purposes, the fuel vapor filling the space in the fuel tank may exhaust into the open air as refuelling proceeds. Hereupon, it becomes necessary to reduce the amount of emission of fuel vapor into the atmosphere in such a case. To achieve this purpose, the fuel in the fuel tank is always sent into the canister irrespective of the internal pressure of the fuel tank until the engine is stopped for refuelling to thereby reduce the amount of fuel vapor remaining in the fuel tank when the lid is opened as far as possible.
To meet the above requirements, the fuel tank passage should communicate with the canister passage by a third communication passage at a place separate from the constant-pressure operating valve and the differential pressure operating valve, and open and close the third communication passage by a third valve which is opened when the engine is in operation. However, provision of such a third communication passage and third valve causes an increase in the number of component parts and also an increase in the size of the entire fuel vapor control valve device because of the need for further extension of the canister passage and the fuel tank passage.
To solve the problems presented above, the invention of this application has for its principal object to always capture fuel vapor in the canister when the engine is in operation without having to provide a third valve separate from the constant-pressure operating valve and the differential pressure operating valve, and for its additional object to avoid an increase in the size of the fuel vapor control valve device in such a case.